A technique for creating a stereoscopically viewed image using a lenticular sheet, which has a large number of lenticular lenses arrayed in parallel to each other, has been known. This is done for example by subdividing R and L viewpoint images, which have been taken from right and left points of view, into lines, and arranging the subdivided lines (stripe images) of the R viewpoint image alternately with the stripe images of the L viewpoint image on the back of the lenticular sheet such that adjoining two stripe images are positioned underneath one lenticular lens. The R and L viewpoint images having a parallax to each other are seen as a stereoscopic image when they are observed through the lenticular lenses by the left and right eyes respectively. It is also known capturing N viewpoint images (N=3 or more), subdividing these images into stripe images, and disposing N lines of these stripe images behind one lenticular lens in order to enhance the stereoscopic effect.
Such stripe images may be printed by a printer on the reverse surface of the lenticular sheet while transporting the lenticular sheet intermittently in a sub scan direction. Immediately after each intermittent transport, a recording head is driven to record the stripe image extending in a main scan direction sequentially onto the lenticular sheet. Thus, at least two kinds of viewpoint images with a parallax to each other are recorded on the reverse surface of the lenticular sheet (see JPA 2000-292871 and JPA 2007-144974).
It sometimes happens that a lenticular sheet is conveyed aslant while multiple viewpoint images are being recorded on the reverse surface of the lenticular sheet. This is referred to as “skew”. In that case, since the longitudinal direction of the lenticular lenses is misaligned with the main scan direction during the image recording, the recording quality will be remarkably degraded. A variety of solutions for preventing the recording quality from being degraded by the skewed lenticular sheet have conventionally been developed.
JPA 2007-076084 discloses a printer that has a photo sensor nearby its recording head so as to detect the position of the lenticular lenses through the photo sensor and adjust the image recording position on the lenticular sheet based on the result of the detected position. Thus, even while the lenticular sheet is on the skew, the image recording position may be adjusted in accordance with this skew.
JPA 1996-137034 discloses a printer that corrects the attitude of the lenticular sheet in advance so as to avoid the skew during the recording. This printer detects a tilt angle of the lenticular lenses relative to the main scan direction and turns the lenticular sheet about an axis perpendicular to a transport plane according to the detection result. For the sake of detecting the tilt angle, a couple of photo sensors aligned in the scanning direction are disposed on a sheet transport track. Each photo sensor outputs a detection signal corresponding to concavities and convexities of the lenticular lenses. The tilt angle of the lenticular lenses may be detected from these detection signals.
However, in a case where the recording position of the viewpoint images is adjusted on the side of the recording head, like in JPA 2007-076084, if the lenticular sheet skews to a large extent, the viewpoint images recorded on the lenticular sheet will be so distorted that the recording quality will be degraded.
On the other hand, where the lenticular sheet is turned to correct its attitude, like in the printer of JPA 1996-137034, the viewpoint images will not have such a distortion. However, it is difficult to determine which one of detection signals output from the two photo sensors (see for example FIGS. 8 to 11) is ahead of or behind the other in phase. Therefore, the printer of JPA 1996-137034 cannot always detect the tilt angle or tilt direction of the lenticular lenses precisely enough to perform the attitude correction with accuracy.
Where a couple of photo sensors are aligned with each other in the main scan direction, like in the printer of JPA 1996-137034, it may be possible to calculate the tilt direction and tilt angle of the lenticular lenses from a time lag between respective times of detection of a leading end of the lenticular sheet by the two photo sensors. However, if the leading end of the lenticular sheet is not parallel to the longitudinal direction of the lenticular lenses, because of manufacture errors or other various factors, it is impossible to detect the exact tilt angle of the lenticular lenses. As a result, it becomes impossible to align the longitudinal direction of the lenticular lenses with the main scan direction.
The present invention has an object to provide a printer that can detect the tilt angle and tilt direction of the lenticular lenses with precision and correct the attitude of the lenticular sheet with high accuracy.